1. Field of the Invention
The present invention relates to a voltage controlled oscillator usable for, for example, generating a local oscillation signal of a wireless communication apparatus, and a PLL circuit and a wireless communication apparatus using the same.
2. Description of the Background Art
A voltage controlled oscillator (VCO) is widely used as a device for generating a local oscillation signal of a wireless communication apparatus. FIG. 15 shows an exemplary structure of a conventional voltage controlled oscillator 600. The conventional voltage controlled oscillator 600 includes inductors 601 and 602, variable capacitance elements 611 and 612, oscillating transistors 603 and 604, a current source 605 and a power supply terminal 606 that receives a voltage Vdd.
The inductors 601 and 602 and the variable capacitance elements 611 and 612 form a parallel resonance circuit. A capacitance value of the variable capacitance elements 611 and 612 changes in accordance with the voltage difference between both of two terminals thereof. Namely, the capacitance value of the variable capacitance elements 611 and 612 changes in accordance with a control voltage Vt applied to a frequency control terminal 607 from an external circuit; and as a result, the resonance frequency of the parallel resonance circuit changes. The oscillation frequency of the conventional voltage controlled oscillator 600 is in the vicinity of the resonance frequency of the parallel resonance circuit. Therefore, the oscillation frequency of the voltage controlled oscillator 600 can be controlled to be a desired value by adjusting the control voltage Vt. The oscillating transistors 603 and 604 are provided for generating a negative resistance to cancel the loss caused by a parasitic resistance component of the parallel resonance circuit and thus to fulfill the oscillation conditions.
The relationship between the control voltage Vt and the oscillation frequency of the conventional voltage controlled oscillator 600 is substantially determined by the characteristics of the variable capacitance elements 611 and 612. Hence, it is desirable that the variable capacitance elements 611 and 612 have a capacitance value which gradually changes over a wide range of control voltage Vt. In other words, it is desirable that the oscillation frequency linearly changes over a wide range of control voltage Vt.
The reason is as follows. When a PLL (phase locked loop) circuit is structured using the conventional voltage controlled circuit 600, the transient response characteristic and the noise band characteristic of the PLL circuit depend on the frequency sensitivity (the ratio of an oscillation frequency change with respect to the control voltage Vt). Therefore, if the frequency sensitivity changes in accordance with the frequency (if the frequency nonlinearly changes), the characteristics of the PLL circuit per se change in accordance with the frequency. In an area where the frequency sensitivity with respect to the control voltage Vt is high, there is a problem that the frequency is changed even by slight noise applied to the frequency control terminal 607, and thus the phase noise characteristic is lowered.
In actuality, however, it is difficult to use variable capacitance elements having a high level of linearity as the variable capacitance elements 611 and 612 in the conventional voltage controlled oscillator 600 formed on a semiconductor substrate. The reason is that it is costly to introduce a special process in order tot form such variable capacitance elements 611 and 612. FIG. 16A shows an exemplary structure of a variable capacitance element using a gate capacitance, which is widely used in CMOS process. FIG. 16B shows a change in the gate capacitance when a reference voltage Vref is applied to a gate of a MOS transistor and a control voltage Vt is applied to a drain and a source of the MOS transistor.
As shown here, in a variable capacitance element using a gate capacitance of a generally used MOS transistor, the capacitance value rapidly changes in the vicinity of a threshold voltage (voltage Vth in FIG. 16B). Therefore, the oscillation frequency also rapidly changes in the vicinity of the threshold voltage. As a result, a PLL circuit including the conventional voltage controlled oscillator 600 has a problem that the transient response characteristic and the noise band characteristic significantly change in accordance with the frequency.
In order to solve the above-described problems, voltage controlled oscillators 700 and 800 are conventionally proposed, in which the linearity of the variable capacitance elements is improved (see, for example, Japanese Laid-Open Patent Publications Nos. 2004-147310 and 2001-352218). FIG. 17 and FIG. 18 respectively show exemplary structures of the conventional voltage controlled oscillators 700 and 800. In FIG. 17 and FIG. 18, substantially identical components to those of FIG. 15 bear the identical reference numerals thereto, and detailed descriptions thereof will be omitted. Since the conventional voltage controlled oscillators 700 and 800 basically operate in the same manner, the conventional voltage controlled oscillator 700 will be described as a representative example.
The conventional voltage controlled oscillator 700 includes a power supply terminal 606 that receives a voltage Vdd, inductors 601 and 602, oscillating transistors 603 and 604, a current source 605, a reference voltage generation section 708, variable capacitance elements 711, 712, 721, 722, 731 and 732, DC cutting capacitive elements 713, 714, 723, 724, 733 and 734, and radio frequency inhibiting resistors 715, 716, 725, 726, 735 and 736.
The variable capacitance elements 711 and 712 and the DC cutting capacitive elements 713 and 714 form a first variable capacitance circuit 710. The variable capacitance elements 721 and 722 and the DC cutting capacitive elements 723 and 724 form a second variable capacitance circuit 720. The variable capacitance elements 731 and 732 and the DC cutting capacitive elements 733 and 734 form a third variable capacitance circuit 730. The capacitance value of the variable capacitance elements 711, 712, 722, 723, 731 and 732 changes in accordance with the reference voltage input to a connection point B of the respective variable capacitance element and the corresponding DC cutting capacitive element and also in accordance with the control voltage Vt applied to the frequency control terminal 607. As a result, the resonance frequency of the parallel resonance circuit changes.
The reference voltage generation circuit 708 controls an output thereof such that the reference voltages which are input to the variable capacitance circuits 710, 720 and 730 are respectively Vref, Vref−Vd and Vref−2Vd. At this point, the characteristics of the variable capacitance circuits 710, 720 and 730 with respect to the control voltage are shifted by the level of a voltage Vdd as shown in FIG. 19. The capacitance value of the parallel resonance circuit, which is a total capacitance value of the three variable capacitance circuits 710, 720 and 730, gradually changes with respect to the control voltage Vt as represented with the one-dot chain line in FIG. 19.
However, the above-described conventional voltage controlled oscillators 700 and 800 need to use a plurality of variable capacitance circuits in order to allow the capacitance value to be changed gradually with respect to the control voltage Vt. In order to allow the capacitance value to be changed gradually over a wider range of control voltage Vt, the number of variable capacitance circuits arranged in parallel needs to be increased. This involves a problem of enlarging the chip area. In addition, since it is difficult to lay out a large number of variable capacitance circuits in parallel on a semiconductor substrate, there is a limit on the number of the variable capacitance circuits. Thus, it is still difficult to allow the capacitance value to be changed gradually over a wide range of control voltage Vt.